Anyway to make a homebrew ultimate ability with charges?

So my idea is to essentially give my players an "Ultimate" skill that can be activated after charging their ultimate gauge. Im just trying to think of a way to implement that into dnd beyond and currently the best thing I can think of is to give them a feat and give it a set amount of charges that they will then fill when meeting certain requirements and once it is "Charge" they use the ability and then reset the charges to 0.

Are there any better ways of going about this idea? The only thing that I don't really like is that since they are considered charges on the feat they start at having full charges and then it goes down instead of starting at 0 and going up.

I don't mind the extra leg work to reset the charges to 0 and have them manually add the points as they go. Just wondering if anyone more experienced has a better way of keeping track of something like this where something has to be done a certain amount of times before something else can be used.

Thanks everyone!

I’m not sure there is a better way although you could have the ability activate when it reaches 0 then it resets the maximum number of charges. For example, if they fill their gauge a certain amount by defeating an enemy you can have their number of charges go down by 1 every time they defeat an enemy. But I’m not actually sure that that’s better.